1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV), and in particular to avoiding cyclic shifting among gears of a transmission while the vehicle corners or turns on a winding road.
2. Description of the Prior Art
A motor vehicle powertrain having an internal combustion engine and an automatic transmission, in which discrete gear ratios are produced, typically has a shift schedule calibrated to schedule downshifts when the engine runs out of reserve power and to schedule upshifts as close as possible to tractive effort crossovers. The shift schedule is calibrated for use in a vehicle that is unloaded and on straight road since the vehicle generally operates in these conditions.
When the vehicle is negotiating a curve or turning a corner, the straight road calibration can cause a condition, wherein the transmission repetitively upshifts and downshifts between gears. Excessive gear shifting occurs when the vehicle begins to decelerate in the turn the driver tips-in, i.e., depresses the accelerator pedal either at a high rate or to a substantial portion of its travel, to maintain vehicle speed. As a result of the tip-in, the transmission downshifts. Because the vehicle begins to accelerate beyond the desired speed after the downshift is completed, the driver tips-out of the throttle causing an upshift to occur. This cyclic gear shifting continues until the vehicle exits the curve.
To avoid this frequent gear shifting, it is conventional to perform the downshift and to prevent a subsequent upshift. Although this procedure mitigates the shift frequency problem, it can lead to driver dissatisfaction due to the downshift that occurs upon entering the turn. Furthermore, the early downshift causes a fuel economy penalty attributable to remaining in the lower gear for longer period, whereas fuel economy is maximized in the higher gear.